Case study: Anoxia tolerance in the Pacific hagfish (Eptatretus stoutii)

Abstract

Pacific hagfish (Eptatretus stoutii) are an agnathan fish capable of surviving 36h of anoxia. This is achieved through a variety of factors. Under normoxic conditions they possess one of the lowest metabolisms recorded among vertebrates. Their systemic heart has a rate of energy consumption so low that it lies within the capacity of anaerobic metabolism. The energy needs of the heart are reduced by an anoxia-induced reduction in heart rate by 50%, partially compensated for by increased stroke volume. This reduces the energy consumption of the heart by up to a quarter during anoxia. The Pacific hagfish also utilize hypometabolism, reducing their metabolic rate by 50%, to conserve energy during anoxia. These features allow the hagfish to avoid energy depletion during anoxia. Their anoxic survival is also aided by a strong tolerance of acidosis. Pacific hagfish can survive a plasma pH of 6.8. Hagfish have a high blood Cl− concentration due to ionoconformation with seawater. This gives the hagfish a large capacity to recover blood pH by exchanging Cl− for HCO3−.